Processes for the cracking of hydrocarbon feedstocks via contact at appropriate temperatures and pressures with fluidized catalytic particles are generically as "fluid catalytic cracking" (FCC). A heavy feed contacts not regenerated catalyst and is cracked to lighter products. Carbonaceous deposits form on the catalyst, thereby deactivating it. The deactivated (spent) catalyst is separated from cracked products, stripped of strippable hydrocarbons and conducted to a regenerator, where coke is burned off the catalyst with air, thereby regenerating the catalyst. The regenerated catalyst is then recycled to the reactor. The reactor-regenerator assembly are usually maintained in heat balance. Heat generated by burning the coke in the regenerator provides sufficient thermal energy for catalytic cracking in the reactor. Control of reactor conversion is usually achieved by controlling the flow of hot regenerated catalyst to the reactor to maintain the desired reactor temperature.
In most modern FCC units, the hot regenerated catalyst is added to the feed at the base of a riser reactor. The fluidization of the solid catalyst particles may be promoted with a lift gas. Mixing and atomization of the feedstock may be promoted with steam, equal to 1-5 wt % of the hydrocarbon feed. Hot catalyst (650.degree. C..sup.+) from the regenerator is mixed with preheated (150.degree.-375.degree. C.) charge stock. The catalyst vaporizes and superheats the feed to the desired cracking temperature usually 450.degree.-600.degree. C. During the upward passage of the catalyst and feed, the feed is cracked, and coke deposits on the catalyst. The coked catalyst and the cracked products exit the riser and enter a solid-gas separation system, e.g., a series of cyclones, at the top of the reactor vessel. The cracked products pass to product separation. Typically, the cracked hydrocarbon products are fractionated into a series of products, including gas, gasoline or naphtha, heavy gasoline or heavy naphtha, light gas oil, and heavy cycle gas oil. Some heavy cycle gas oil may be recycled to the reactor. The bottom's product, a "slurry oil", is conventionally allowed to settle. The catalyst rich solids portion of the settled product may be recycled to the reactor. The clarified slurry oil is a heavy product.
The "reactor vessel" into which the riser discharges primarily separates catalyst from cracked products and unreacted hydrocarbons and permits catalyst stripping.
Older FCC units use some or all dense bed cracking. Down flow operation is also possible, in which case catalyst and oil are added to the top of a vertical tube, or "downer," with cracked products removed from the bottom of the downer. Moving bed analogs of the FCC process, such as Thermofor Catalytic Cracking (TCC) are also known.
Further details of FCC processes can be found in: U.S. Pat. Nos. 4,093,537, 4,118,337, 4,118,338, 4,218,306 (Gross et al.); 4,444,722 (Owen); 4,459,203 (Beech et al.); 4,639,308 (Lee); 4,675,099, 4,681,743 (Skraba) as well as in Venuto et al., Fluid Catalytic Cracking With Zeolite Catalysts, Marcel Dekker, Inc. (1979). These patents and publication are incorporated herein by reference.
Conventional FCC catalysts usually contain acidic zeolites such as Rare Earth Y (REY), Dealuminized Y (DAY), Ultrastable Y (USY), Rare Earth Containing Ultrastable Y (RE-USY), and Ultrahydrophobic Y (UHP-Y).
Typical FCC catalyst particle diameters range from 20 to 150 microns, with an average diameter of 60 to 80 microns.
Catalysts used in moving bed catalytic cracking units (e.g. TCC units) are in the form of spheres, pills, beads, or extrudates, and can have a diameter ranging from 1 to 6 mm.
Although many advances have been made in both the catalytic cracking process, and the process produces close to half of the gasoline consumed in the United State, some problem areas remain.
The FCC gasoline pool has a fairly high octane number, but there is an ever increasing demand for more octane. The FCC gasoline is of good quality, but the volatility is somewhat higher than desired. End point restrictions have become more stringent to meet demands for clean burning fuel.
FCC gasoline, unlike reformate, has relatively large amounts of sulfur and olefins. Any attempts to reduce gasoline sulfur content by hydrotreating would also reduce gasoline octane number. The FCC heavy naphtha is especially troublesome re. sulfur, because the sulfur content increases with boiling point.
An additional concern in many areas is that there is not enough gasoline. This will an even more serious problem as new environmental regulations come into force, which may restrict the gasoline 90% boiling point to about 300.degree. F. in some areas.
It would be beneficial if refiners had an efficient way to convert streams which are too heavy, or of poor quality, into products which could enter the refinery gasoline pool. FCC heavy naphtha is not as valuable as lighter naphtha.
Coker heavy gasoline is too heavy and too reactive to permit its inclusion in the gasoline pool.
Hydrocracked naphtha may not be suitable for blending in the gasoline pool, and may require further processing.
Virgin naphtha has a low octane number, and usually must be hydrotreated and reformed in a platinum reformer to permit addition to the refinery gasoline pool.
Refiners have used the FCC process as a way to upgrade to some extent one or more of these streams. Some methods of upgrading naphtha boiling range streams in FCC risers will be reviewed.
Coker naphtha has been added to an FCC riser, with much of it converted to coke.
U.S. Pat. No. 4,218,306, Gross et al., taught improving naphtha octane in a very bottom portion of a riser reactor, by contact with freshly regenerated catalyst.
In U.S. Pat. No. 4,832,825 an FCC light naphtha was recycled to the base of an FCC riser, to contact hot regenerated catalyst. The naphtha recycle was reported to double the production of light olefins.
In U.S. Pat. No. 3,847,793, Schwartz, which is incorporated herein by reference, C3 and C4 hydrocarbons, alone or in combination with a recycled gasoline fraction, were added to a dense bed reactor at the top of a riser reactor. This process is interesting because, although conducted in, or rather just downstream of, an FCC riser reactor, the conversion of C3/C4 and naphtha was apparently due solely to the action of ZSM-5 catalyst present in the inventory. The large pore cracking catalyst acted solely as a heat sink.
In the '793 process, the FCC catalyst inventory contained large amounts of ZSM-5, which retained activity during riser cracking. A heavy feed, such as a gas oil, was added to the base of a riser reactor and cracked. Resid, injected near the top of the riser, deactivated the large pore cracking catalyst. The riser discharged into a dense bed, where C3/C4, and optionally recycled gasoline, were added. The large pore catalyst provided a heat sink, while the ZSM-5 converted olefins and paraffins to lower boiling olefins and alkylbenzene and reduced the average weight of the alkylbenzene.
The '793 patent examples showed cracking of gasoline in a bench scale FCC unit at 960.degree. F., 5 C/O, 0.2 WHSV, with catalyst coked to contain 2.66 to 2.7 wt % coke. The catalyst contained relatively large amounts of ZSM-5, e.g., 5% ZSM-5, 15% REY, all in a matrix. Roughly 60% yields of gasoline were obtained, with conversion of roughly 10 wt % of the feed to (coke and 450.degree. F.+ material).
The '793 patent used the heat energy, but little or none of the catalytic activity, in "top of the riser" FCC catalyst. In U.S. Pat. No. 4,032,432, Owen, both the thermal energy and catalytic activity of this catalyst were used to oligomerize or cyclicize vapors from an FCC main column boiling below a C6+ gasoline stream. The catalyst specified was a mixture of large pore zeolite cracking catalyst and a smaller pore zeolite. In the process, a vapor fraction, having an average molecular weight of about 40, was heated, then charged to the base of a lift tube embedded in the reactor vessel containing the FCC spent catalyst stripper. The top of the lift tube discharged into a cyclone, having a vapor outlet isolated from the vapor products of the FCC reaction. The process reduced the requirements of the light ends recovery process by up to about 30%. The '432 patent thus showed a good way to convert light, normally vaporous hydrocarbons into heavier liquids, and to unload to some extent the light ends recovery facilities associated with FCC plants. The process did not act upon any normally liquid stream from the cracking unit.
Quenching of FCC risers with various liquid recycle streams is suggested in several patents. Light and heavy cycle oils, FCC naphtha, and water have all been proposed as quench liquids. Such quench fluids are considered relatively inert.
Processing of FCC heavy naphtha in a separate reactor, sharing a common regenerator with the FCC reactor, has been proposed.
We reviewed the problems of improving the quality of FCC gasoline, and trying to make more of it from various streams available in refineries, and found no completely satisfactory solution. The state of the art either did too much, or too little, to these streams.
Simply recycling naphtha to the base of an FCC riser, upstream of the point of fresh feed addition, or recracking the naphtha in a separate reactor with freshly regenerated cracking catalyst severely overcracks the naphtha. Large amounts of light olefins are produced, but gasoline boiling hydrocarbons are lost, a loss exceeding the potential yield of gasoline from alkylatable olefins produced during overcracking. The end point of gasoline can increase during recracking, due to thermal reactions. An additional problem is that the large amount of light ends produced will usually overwhelm the capacity of the FCC wet gas compressor and gas plant.
We wanted to recrack heavy naphtha, and similar refinery streams, without overcracking it. We wanted to decrease the FCC gasoline olefinicity and optionally the volatility of the gasoline. We wanted to decrease significantly the sulfur content and the 90% boiling point of heavy gasoline.
We also wanted a way to upgrade other relatively light streams, that is lighter than gas oil, without subjecting them to the severe FCC reaction conditions. We felt that something less severe than FCC processing was the optimum way to handle these relatively light liquids.
We discovered, in the FCC process, a catalyst with ideal properties for our purpose--FCC catalyst discharged from a riser reactor. This material is considered inactive or dead, but still retains considerable activity, and quite a lot of thermal energy. It has ideal properties for catalytic upgrading of FCC heavy naphtha, virgin naphtha and similar streams, but its potential has generally been ignored. Schwartz, in U.S. Pat. No. 3,847,793 realized some of the potential in FCC catalyst. He recognized its considerably residual activity and its value as a heat source. Schwartz, however, used it only for its heating value, preferring to eliminate its catalytic activity by adding a resid or equivalent heavy feed to essentially completely deactivate it with coke, while allowing the ZSM-5 present in the equilibrium catalyst to do its work.
We realized that spent FCC catalyst, because of its lower temperature, as compared to freshly regenerated FCC catalyst, does less thermal cracking. We believed that heavy naphtha could be converted by post-riser FCC catalyst, even using FCC catalyst which contained none, or only modest amounts of ZSM-5, and that coking of the FCC catalyst was neither essential nor beneficial. We believed that "top of the riser" FCC catalyst could crack heavy naphtha, without producing large amounts of light ends or high end point material.
We discovered a way to recrack light liquid hydrocarbon streams, using "top of the riser" cracking catalyst, without increasing catalyst traffic in the FCC reactor or catalyst stripper. In a preferred embodiment, our process and apparatus operates in unison with the FCC main column without significantly increasing vapor traffic in the column. We also discovered a way to continuously operate a separate fluidized bed reactor for naphtha recracking, without an additional catalyst regenerator, and/or a catalyst stripper. We were able to integrate an additional cracking process with the FCC process, but in a way which did not reduce the FCC unit's capacity nor impair its reliability.